Optical near-field microscopy is based upon the measurement of scattered light at a near-field probe which is affected by optical near-field interaction between the near-field probe and a specimen. To achieve high local resolution capacity, a probe tip, for example, is used as a near-field probe, as known from atomic force microscopy. The near-field probe is illuminated by focused light to generate the scattered light. Focusing should be as free from chromatic errors as possible, to avoid measurement errors with illumination at different wavelengths, e.g. in the visible or middle infra-red spectrum. Hitherto, transmitting optical components, e.g. optical lenses, by means of which the requisite suppression of chromatic errors due to material dispersion in the components cannot be achieved, or can only be achieved to an insufficient extent, have been used for focusing in near-field microscopy.
To avoid chromatic errors caused by material dispersion, T. Nakano et al. proposed focusing the illuminating radiation with reflective optical components, e.g. rotation-symmetrical concave minor lenses (“Optik”Vol. 94, 1993, pp. 159- 162). A reflector in the shape of a paraboloid was, for example, used as a concave mirror lens, the paraboloid axis of which is perpendicular to the surface of the specimen under examination. This geometry produces irradiation of the near-field probe more or less perpendicular to the surface, as is usual with glass lenses in light-optical microscopy. A disadvantage of conventional reflectors is that the direction of polarization of the illuminating radiation is more or less transverse, but a light field strength oriented perpendicular to the specimen is preferred for the effect of optical near-field interaction with the specimen. If inclination of the conventional concave mirror lens is introduced, further disadvantages arise from the space required by the near-field probe and the outlay with an adjustment being free of coma aberration.
In the “Japanese Journal of Applied Physics”, Vol. 39, 2000, pp. 888-891, K. Ueyanagi et al. proposed providing a read head for the near-field optical readout of an optical data memory with a focusing reflector. The reflector is a SIM (solid immersion mirror) with a surface in the form of a semi-paraboloid. The reflector is aligned so that the paraboloid axis of the cut paraboloid extends in parallel to the surface of the optical data memory, and illuminating radiation which is directed along the paraboloid axis on to the surface is focused on to the surface of the optical data memory. The use of the SIM described by K. Ueyanagi is restricted to reading optical data memories. Use in optical near-field microscopy is precluded, as the SIM neither provides enough space for locating the near-field probe in the focal point of the mirror nor for monitoring its operation.